1. Field of the Invention
This invention relates generally to a method for evaluating the performance of a drilling motor under downhole conditions. In particular, it relates to a method for evaluating the power output of a drilling motor and using this evaluation data to optimize the performance of the motor during downhole drilling operations.
2. Description of the Related Art
A drilling motor is a mechanical tool based on a progressive cavity device similar to the positive displacement pump first reported by Moineau and is used to drive a drill bit for directional drilling of wells. A drilling motor operates by translating the flow of pressurized drilling fluid (mud) into the rotation of a helical rotor, within a similar lobed-type stator. FIG. 1 shows the cross-section of a typical drilling motor used in the context of the present invention. Drilling fluid flows through area 10, causing the helical rotor 11 to rotate around the lobes 12 in the stator 13. The motor has a maximum mechanical power output. When the motor approaches this maximum power output, any additional hydraulic power supplied to the motor is dissipated by deformation of the stator lobes which are typically formed of a rubber compound. A deformed stator in the motor results in a reduced rate at which the drill bit, connected to the motor, penetrates the formation.
In order to use the motor optimally in terms of the translation of hydraulic power into mechanical power at the drill bit, and to decrease the chances for stator deformation, it is necessary to know the downhole characteristics that affect the power output of the motor under downhole conditions. The downhole characteristics of interest include weight-on-bit (WOB), torque, motor shaft speed and the pressure drop across the motor's power section. Measurements of these characteristics are preferably made downhole and in a continuous manner so that they are representative of actual values. Downhole measurements of such characteristics are usually transmitted uphole, by a measurment-while-drilling (MWD) tool, for processing and display at the surface in substantially real-time. Based on calculations from these measurements, operators can adjust drilling parameters and, therefore, maximize the mechanical power output of the motor and the rate of penetration of the drill bit while reducing wear on the stator to a minimum. Typically, the power output of the motor is reported as a function of the pressure drop across the motor. However, this power data is most often generated for the motor under surface conditions and therefore may be an inaccurate representation of the motor characteristics in downhole environments.
By utilizing calibration techniques, it is possible to determine the power output of the motor under downhole conditions and thus use such data to optimize the operating parameters of the motor. Furthermore, by evaluating the power output of the motor over a period of time, it is possible to determine any degradation of motor performance and indicate a suitable stage at which the motor is no longer economical to operate.
U.S. Pat. No. 5,368,108 describes one method for optimizing the performance of a downhole drilling motor. This patent describes a method for determining the maximum power output of a downhole drilling motor and the hydraulic power that is input to the motor. Hydraulic power input and maximum power output are plotted versus one another to obtain a characteristic curve. The mechanical power output is proportional to downhole torque on the drill bit and to the rotary speed (RPM) of the bit. Torque and RPM are measured continuously downhole and the measurements transmitted to the surface. The hydraulic power input to the motor is a function of pressure drop across the motor and the flow rate therethrough. A plot of the mechanical power output with increasing hydraulic power input has a predictable shape, assuming a constant flow rate. The optimum power output occurs when the slope of this plotted curve is no longer positive, that is, the value thereof reaches a maximum and will shortly begin to decline.
The technique described in the '108 patent uses the power curve to obtain the optimum power output, and thus the optimum torque value. The optimum power output can be compared with the theoretical value from motor specifications to determine the effects of wear and temperature on the motor performance. The optimum downhole weight-on-bit is computed for the optimum torque value since there is a linear relationship between downhole torque and weight-on-bit for a given lithology. Such optimum weight-on bit is computed in real time, together with a representation of the power curves, to indicate the position on such curves for the driller. The optimum rate of penetration can be determined, since rate of penetration is a linear function of the mechanical power output of the motor. The optimum mechanical power output has a corresponding hydraulic power input from which an optimum standpipe pressure can be determined.
Although the method of the '108 patent is effective, this method still requires the task of actually taking downhole measurements such as motor torque and motor RPM. This method also requires additional downhole equipment to take the measurements. The present invention is a procedure for predicting the power output by the motor from measurements taken at the surface instead of downhole. This information is useful in determining motor performance and assessing motor deterioration.